The story about how secure boot for Windows 8, part of UEFI, will hinder the use of non-signed binaries and operating systems, like Linux, has registered at Redmond as well. The company posted about it on the Building Windows 8 blog - but didn't take any of the worries away. In fact, Red Hat's Matthew Garrett, who originally broke this story, has some more information - worst of which is that Red Hat has received confirmation from hardware vendors that some of them will not allow you to disable secure boot.

You mix things up. We were taking about the HW limiting the OS, not about an OS that is limited to specific HW.

Furthermore, part of "EFI" is the "E" for extensibel. Of course the are specific Apple add-ons only availabel on Macs e. g. Target Disk Mode, the ability to ad-hoc boot from a different device, Target Display Mode, etc.. That still doesn't mean that Apple's HW engineers somehow try to create an incompatible EFI implementation. This is pure and blind speculation from your side.
If they wanted to create a "walled garden HW", they could have used OpenFirmware with x86, which is very uncommon in the x86 world. It's in Apple's own interest to be compatible with standard x86 HW which includes EFI compatibility